The closed system administration of anesthetics requires complete rebreathing of anesthetic mixtures. Because a patient consumes oxygen and produces carbon dioxide, the exhaled gas has had part of its oxygen extracted and replaced by carbon dioxide. Carbon dioxide must be removed and oxygen added to the gas mixture before it is recirculated to the patient. Similarly in semi-closed systems there is partial rebreathing and possible accumulation of carbon dioxide must be monitored.
It has been conventional over the years to use alkalies contained in canisters in fluid communication with the anesthetic administration system to remove carbon dioxide from the gases being administered to the patient (including anesthetic and oxygen). Absorbtion of carbon dioxide by such alkalies is essentially a chemical reaction i.e. neutralization. Carbonic acid is formed by the combination of carbon dioxide with water. The base is the absorbent and at the present time the only satisfactory absorbents are the hydroxides of the alkaline metals, potassium, sodium, lithium and of the alkaline earths: barium, stratum, calcium and magnesium. The hydroxides of the alkaline metals are more active chemically than the alkaline earths, but are very caustic and highly hygroscopic. As a result, mixtures of these compositions have been developed for clinical purposes, for example soda lime, which is basically porous limestone granules that are activated for carbon dioxide absorption by the addition of caustics i.e. the alkaline metals. More particularly soda lime is a mixture of sodium, potassium and calcium hydroxides which, by ionization furnish OH-ions. Sodium hydroxide and potassium hydroxide make up about 4.5% of the mixture and calcium hydroxide about 95%. Small amounts of silica are added to make the mixture hard and to minimize fragmentation and alkaline dust formation. In soda lime, silica amounts to about 0.2%. Another commonly used absorbent is baralyme. This is a mixture of about 20% barium hydroxide and about 80% calcium hydroxide.
Until now, chemical indicators have been incorporated into the granules of absorbents. These are dyes that change colour when exposed to different pH values resulting as carbon dioxide becomes "absorbed" and neutralizes the basic absorbents. The acid form of the indicator has one colour and the basic form another. Indicators such as ethyl violet or Clayton yellow are dyes that change colour at a pH below 12. As carbonic acid accumulates the indicator on the absorbent granules will change colour visibility. In baralyme, indicators such as mimosa (pink) and ethyl violet are used.
The problem with the use of such indicators is that they must be observed for indicator change during active use. Otherwise, as they stand, when the anesthetic delivery system is inactive, they tend to revert to their original colour. Thus, the remaining capacity of absorption of the absorbent becomes unknown. With reuse of the system, the indicator will change colour again quickly, particularly where there was little remaining capacity of absorption in the absorbent.
Even when indicators are used in absorbents, exhaustion of the absorbent and corresponding incomplete carbon dioxide removal may be recognized only upon the patient developing hypertension or other visible signs.
Patents of general background interest to the subject matter of the present invention include U.S. Pat. No. 4,457,253 of Manske issued Jul. 3, 1984 which teaches a capillary tube temperature indicator, Canadian Patent No. 610,761 of Davidson et al issued Dec. 20, 1960 which teaches a wax spot for packaged frozen foods which wax spot melts at a predetermined temperature and disappears from visibility indicating that the food may have been previously thawed, U.S. Pat. No. 4,326,514 of Eian issued Apr. 27, 1982 that describes a respirator carbon dioxide absorbent indicator in the form of a colorimetric indicator in sheet form contained in a respirator canister, Canadian Patent No. 1,100,419 of Jones issued May 5, 1981 which teaches an end of service indicator for organic vapour/gas respirator cartridges wherein a color change agent is contained in the absorbent bed of the respirator; Canadian Patent No. 1,166,941 of De Blauwe issued May 8, 1984 which teaches a thermochromic composition used as a warning indicator that a certain maximum safety temperature has been achieved and Canadian Patent No. 1,141,990 of Sewell et al issued Mar. 1, 1983 which teaches an electronic indicator to monitor the capacity of a carbon dioxide absorbent to absorb carbon dioxide.
It is an object of the present invention to provide a more reliable indicator and method of indicating exhaustion or developing exhaustion of an absorbent in such anesthetic gas administration systems.